1. Technical Field of the Invention
The present invention relates to the coding and decoding of digital data for transmission over a communications channel and, in particular, to a system decoder utilizing soft information outputs to minimize error rates in the transmitted digital data.
2. Description of Related Art
There exist many applications where large volumes of digital data must be transmitted and received in a substantially error free manner. In telecommunications systems, in particular, it is imperative that the reception of digital data be accomplished as reliably as is possible. Reliable communication of digital data is difficult, however, because the communications channels utilized for data transmission are plagued by error introducing factors. For example, such errors may be attributable to transient conditions in the channel (like noise or multi-path fading). The influence of such factors results in instances where the digital data is not transmitted properly or cannot be reliably received.
Considerable attention has been directed toward discovering methods for addressing the instances of errors which typically accompany data transmission activities. For example, it is well known in the art to employ forward error correction (FEC) codes and other means to locate, counteract, correct and/or eliminate these errors. The data stream is thus encoded in accordance with a plurality of predefined codewords established in a codebook for that particular encoding scheme. Once encoded, the random errors introduced therein during transmission are relatively easily located and corrected during a corresponding decoding process using well known mathematical processes.
The codewords output from the encoder are transmitted over a communications channel and corrupted to some degree by noise to create a vector. During decoding, the received vector comprising the encoded data (perhaps including errors) is compared against each of the plurality of codewords in the codebook for the specific encoding process used. The codeword closest to the received vector is selected, and its corresponding data word is output from the decoder. This output is often referred to as the hard information output.
In many coding systems, a decoder additionally produces soft (or side) information outputs to help another decoder identify, and perhaps correct, introduced errors. For example, in one scenario used in the Global System for Mobile (GSM) communication, an inner decoder comprising an equalizer generates a soft information output derived from path metric differences, and an outer decoder comprising an error control decoder utilizes the output soft information to detect and correct introduced errors. In another scenario used in the PRISM-TDMA communications system, the inner decoder comprises an improved multiband excitation (IMBE) error control decoder generating estimates of the number of channel errors, and the outer stage decoder comprises a speech decoder which utilizes the output error estimates in determining whether to discard data.
Soft information outputs have historically been generated by the decoder in conjunction with the selection of the closest codeword and its associated hard information output. The reliability information comprising the soft information output is calculated for each individual symbol (bit) within the hard information output. Accordingly, in such decoders the reliability of each symbol (bit) within the hard information output vector is derived without taking into consideration either the remaining symbols within that hard information output vector or any other considered codewords (and associated hard information output vectors). This is achieved by comparing the probability of the received data given a bit with a logical value of one was transmitted to the probability of the received data given a bit with a logical value of zero was transmitted.